Nervous System Nervous System

The nervous system is composed of The nervous system is composed of billions of neurons with long, billions of neurons with long, interconnecting processes that form interconnecting processes that form complex integrated electrochemical complex integrated electrochemical circuits. It is through these neuronal circuits. It is through these neuronal circuits that animals experience circuits that animals experience sensations and respond appropriately. sensations and respond appropriately.

Basic Sensory and Motor Basic Sensory and Motor Functions:Functions:

The The peripheral nervous systemperipheral nervous system (PNS) is (PNS) is formed by neurons of the cranial and formed by neurons of the cranial and spinal nerves. The spinal nerves. The central nervous central nervous systemsystem (CNS) is formed by neurons of the (CNS) is formed by neurons of the spinal cord, brain stem, cerebellum, and spinal cord, brain stem, cerebellum, and cerebrum.cerebrum.

Evidence of disease in other body systems Evidence of disease in other body systems may be associated with inflammatory, may be associated with inflammatory, metabolic, toxic, or metastatic neoplastic metabolic, toxic, or metastatic neoplastic disorders of the nervous system. External disorders of the nervous system. External signs of traumatic or toxic exposure may signs of traumatic or toxic exposure may support these mechanisms of disease.support these mechanisms of disease.

The neurologic examination consists of The neurologic examination consists of evaluation of the following:evaluation of the following:

1) the head.1) the head. 2) the gait. 2) the gait. 3) the neck and thoracic limbs. 3) the neck and thoracic limbs. 4) the trunk, pelvic limbs, anus, and tail. 4) the trunk, pelvic limbs, anus, and tail.

Initially, an attempt should be made to Initially, an attempt should be made to relate all deficits to a focal anatomic relate all deficits to a focal anatomic lesion.lesion.

Mechanisms of Disease:Mechanisms of Disease:

Disease processes affecting the nervous Disease processes affecting the nervous system may be congenital or familial, system may be congenital or familial, infectious or inflammatory, toxic, infectious or inflammatory, toxic, metabolic, nutritional, traumatic, vascular, metabolic, nutritional, traumatic, vascular, degenerative, neoplastic, or idiopathic.degenerative, neoplastic, or idiopathic.

Infections of the nervous system are due Infections of the nervous system are due to specific viruses, fungi, protozoa, to specific viruses, fungi, protozoa, bacteria, rickettsia, and algae bacteria, rickettsia, and algae

Rabies Rabies

Rabies is an acute viral encephalomyelitis that principally Rabies is an acute viral encephalomyelitis that principally affects carnivores, although it can affect any mammal affects carnivores, although it can affect any mammal including food animals. including food animals.

Rabies is most often transmitted by the bite of infected Rabies is most often transmitted by the bite of infected wild animals including skunks, raccoons and foxes.wild animals including skunks, raccoons and foxes.

Bats are the important species in which carriers are Bats are the important species in which carriers are known to occur known to occur

Rabies is endemic in Texas.Rabies is endemic in Texas.

A raccoon with rabies. Courtesy of Dr.

Thomas Lane

Etiology and Epidemiology:Etiology and Epidemiology:

Rabies is caused by lyssaviruses in the Rabies is caused by lyssaviruses in the Rhabdovirus family. Lyssaviruses are Rhabdovirus family. Lyssaviruses are usually confined to 1 major reservoir usually confined to 1 major reservoir species in a given geographic area, species in a given geographic area, although spillover to other species is although spillover to other species is common.common.

Transmission and PathogenesisTransmission and Pathogenesis

Transmission is almost always by Transmission is almost always by introduction of virus-laden saliva into the introduction of virus-laden saliva into the tissues, usually by the bite of a rabid tissues, usually by the bite of a rabid animal. Although much less likely, it is animal. Although much less likely, it is possible for virus from saliva, salivary possible for virus from saliva, salivary glands, or brain to cause infection by glands, or brain to cause infection by entering the body through other fresh entering the body through other fresh wounds or through intact mucous wounds or through intact mucous membranesmembranes

Clinical signs Clinical signs

The most reliable signs, The most reliable signs, regardless of species, are regardless of species, are acute behavioral changes and acute behavioral changes and unexplained progressive unexplained progressive paralysis. Behavioral changes paralysis. Behavioral changes may include sudden anorexia, may include sudden anorexia, signs of apprehension or signs of apprehension or nervousness, irritability, and nervousness, irritability, and hyperexcitability (including hyperexcitability (including priapism). The animal may priapism). The animal may seek solitude. Ataxia, altered seek solitude. Ataxia, altered phonation, and changes in phonation, and changes in temperament are apparent temperament are apparent

Clinical signs cont.Clinical signs cont. Due to the vagueness of signs and lack Due to the vagueness of signs and lack

of consideration of rabies as a potential of consideration of rabies as a potential diagnosis, cattle handlers, veterinarians diagnosis, cattle handlers, veterinarians and other people in contact are often and other people in contact are often exposed to the virus. During the active exposed to the virus. During the active course of the disease, the virus is course of the disease, the virus is excreted at high levels in saliva. excreted at high levels in saliva.

Cattle with furious rabies can be Cattle with furious rabies can be dangerous, attacking and pursuing dangerous, attacking and pursuing humans and other animals. Lactation humans and other animals. Lactation ceases abruptly in dairy cattle. The ceases abruptly in dairy cattle. The usual placid expression is replaced by usual placid expression is replaced by one of alertness. The eyes and ears one of alertness. The eyes and ears follow sounds and movement. follow sounds and movement.

Diagnostic Diagnostic

Rabies testing should be done by a Rabies testing should be done by a qualified laboratory, designated by the qualified laboratory, designated by the local or state health department.local or state health department.

Fluorescent antibody testing of the brain, Fluorescent antibody testing of the brain, the presence of Neigri bodies on the presence of Neigri bodies on histopathology. histopathology.

ControlControl

Vaccines approved for use in cattle, sheep Vaccines approved for use in cattle, sheep and horses are available and should be and horses are available and should be seriously considered for high-risk seriously considered for high-risk exposures or high-value animals. Due to exposures or high-value animals. Due to costs of vaccine and relative low incidence costs of vaccine and relative low incidence rates, vaccination is not routinely used in rates, vaccination is not routinely used in most cattle herds. most cattle herds.

Polioencephalomalacia (PEM)Polioencephalomalacia (PEM)

is an important neurologic disease of is an important neurologic disease of ruminants that is seen worldwide. Cattle, ruminants that is seen worldwide. Cattle, sheep, goats, deer, and camelids are sheep, goats, deer, and camelids are affected. affected.

Historically, PEM has been associated Historically, PEM has been associated with altered thiamine status, but more with altered thiamine status, but more recently an association with high sulfur recently an association with high sulfur intake has been observed. intake has been observed.

Etiology, Pathogenesis, and Etiology, Pathogenesis, and EpidemiologyEpidemiology

The disease is seen sporadically or as a herd The disease is seen sporadically or as a herd outbreak. In general, younger animals are more outbreak. In general, younger animals are more frequently affected than adults. Animals on high frequently affected than adults. Animals on high concentrate diets are at higher risk, but pastured concentrate diets are at higher risk, but pastured animals also develop PEM..animals also develop PEM..

Thiamine inadequacy can be caused by such Thiamine inadequacy can be caused by such factors as acute dietary deficiency of thiamine or factors as acute dietary deficiency of thiamine or ingestion of plant thiaminases or thiamine ingestion of plant thiaminases or thiamine analogs. analogs.

Clinical Findings:Clinical Findings:

PEM may be acute or subacute. Animals PEM may be acute or subacute. Animals with the acute form manifest blindness, with the acute form manifest blindness, recumbency, tonic-clonic seizures, and recumbency, tonic-clonic seizures, and coma. Those with a longer duration of coma. Those with a longer duration of acute signs have poorer responses to acute signs have poorer responses to therapy and higher mortality. Animals with therapy and higher mortality. Animals with the subacute form initially separate from the subacute form initially separate from the group, stop eating, and display the group, stop eating, and display twitches of the ears and face.twitches of the ears and face.

LesionsLesions

Acutely affected animals may have brain Acutely affected animals may have brain swelling with gyral flattening and coning of swelling with gyral flattening and coning of the cerebellum due to herniation into the the cerebellum due to herniation into the foramen magnum. Slight yellowish foramen magnum. Slight yellowish discoloration of the affected cortical tissue discoloration of the affected cortical tissue may be present. may be present.

Lesions cont.Lesions cont.

Polioencephalomalacia. Transverse section of the dorsal parietal cortex of a Polioencephalomalacia. Transverse section of the dorsal parietal cortex of a feedlot steer with PEM viewed under ambient illumination. Affected feedlot steer with PEM viewed under ambient illumination. Affected segments of the cortical gray matter have a yellowish discoloration and segments of the cortical gray matter have a yellowish discoloration and irregular contours. Courtesy of Daniel H. Gould irregular contours. Courtesy of Daniel H. Gould

TreatmentTreatment

The treatment of choice regardless of The treatment of choice regardless of cause is thiamine. Therapy must be cause is thiamine. Therapy must be started early in the disease course for started early in the disease course for benefits to be achieved. If brain lesions benefits to be achieved. If brain lesions are particularly severe or treatment is are particularly severe or treatment is delayed, full clinical recovery may not be delayed, full clinical recovery may not be possible. The dosage of thiamine is 10-20, possible. The dosage of thiamine is 10-20, mg/kg, IM or SC, tid. Initial treatment may mg/kg, IM or SC, tid. Initial treatment may be administered IV. be administered IV.

Listeriosis Listeriosis

Listeriosis, a disease of the central Listeriosis, a disease of the central nervous system, is caused by the  nervous system, is caused by the  bacterium bacterium Listeria moncytogenes.  Listeria moncytogenes. 

This bacterium can live almost anywhere--This bacterium can live almost anywhere--in soil, manure piles, and grass.  in soil, manure piles, and grass. 

Listeriosis is common in cattle, sheep and Listeriosis is common in cattle, sheep and goats and can occur in pigs, dogs, and goats and can occur in pigs, dogs, and cats, some wild animals, and humans. cats, some wild animals, and humans.

EtiologyEtiology

Listeria moncytogenes.Listeria moncytogenes. Gram-positive Ovoid to rod-shaped Facultative anaerobe Acid but not gas producing from glucose Can multiply at temperatures between ~0 and 45°C

Reservoirs

L. monocytogenes. – decaying vegetation – soils – animal and human feces – sewage – silage – waterIt has been isolated from a wide range ofretail foods.

Pathogenesis:Pathogenesis:

ListeriaListeria organisms that are ingested or organisms that are ingested or inhaled tend to cause septicemia, inhaled tend to cause septicemia, abortion, and latent infection. Those that abortion, and latent infection. Those that gain entry to tissues have a predilection to gain entry to tissues have a predilection to localize in the intestinal wall, medulla localize in the intestinal wall, medulla oblongata, and placenta or to cause oblongata, and placenta or to cause encephalitis via minute wounds in bucal encephalitis via minute wounds in bucal mucosa.mucosa.

Clinical Findings:Clinical Findings:

Encephalitis is the Encephalitis is the most readily recognized most readily recognized form of listeriosis in form of listeriosis in ruminants. It affects all ruminants. It affects all ages and both sexes, ages and both sexes, sometimes as an sometimes as an epidemic in feedlot cattle epidemic in feedlot cattle or sheep. The course in or sheep. The course in sheep and goats is rapid, sheep and goats is rapid, and death may occur 24-and death may occur 24-48 hr after onset of signs; 48 hr after onset of signs; however, the recovery however, the recovery rate can be up to 30% rate can be up to 30% with prompt, aggressive with prompt, aggressive therapytherapy

Lesions:Lesions:

In listeric encephalitis, there are few In listeric encephalitis, there are few gross lesions except for some congestion gross lesions except for some congestion of meninges. Microscopic lesions are of meninges. Microscopic lesions are confined primarily to the pons, medulla confined primarily to the pons, medulla oblongata, and anterior spinal cord. oblongata, and anterior spinal cord.

Lesions of listerial encephalitis (microabscessation) in the midbrain of a cow. Courtesy of the Department of Pathobiology, University of Guelph

DiagnosticDiagnostic

Listeriosis is confirmed only by isolation Listeriosis is confirmed only by isolation and identification of and identification of LL monocytogenesmonocytogenes . . Specimens of choice are brain from Specimens of choice are brain from animals with CNS involvement and animals with CNS involvement and aborted placenta and fetus aborted placenta and fetus

Treatment and Control:Treatment and Control:

L monocytogenes is susceptible to penicillin (the L monocytogenes is susceptible to penicillin (the drug of choice), ceftiofur, erythromycin, and drug of choice), ceftiofur, erythromycin, and trimethoprim/sulfonamide. High doses are trimethoprim/sulfonamide. High doses are required because of the difficulty in achieving required because of the difficulty in achieving minimum bactericidal concentrations in the brain. minimum bactericidal concentrations in the brain.

Penicillin G should be given at 44,000 U/kg body Penicillin G should be given at 44,000 U/kg body wt, IM, daily for 1-2 wk; the first injection should wt, IM, daily for 1-2 wk; the first injection should be accompanied by the same dose given IV be accompanied by the same dose given IV

Zoonotic RiskZoonotic Risk

Whether animals serve as a reservoir of Whether animals serve as a reservoir of infection for humans may be questioned, infection for humans may be questioned, because because ListeriaListeria organisms have been isolated organisms have been isolated from feces of a significant number of apparently from feces of a significant number of apparently healthy people as well as other animals. healthy people as well as other animals.

LL monocytogenesmonocytogenes can be isolated from milk of can be isolated from milk of mastitic, aborting, and apparently healthy cows.mastitic, aborting, and apparently healthy cows.

ListeriaListeria also have been isolated from milk of also have been isolated from milk of sheep, goats, and women. sheep, goats, and women.